Change of nitrate concentration in groundwater under nature reserves

What do you see: the trend in the average nitrate concentration in the upper meter of groundwater in nature reserves for the period 1989-2014. The map shows the decrease (in %) in nitrate concentration per location between 1989 and 2014 (a green/yellow ball: nitrate has fallen; a red/purple ball: nitrate has risen). The only input of N and S in these areas takes place via air, via atmospheric deposition. As a result of (inter)national measures, N and S emissions and depositions have decreased and this is reflected in the trends in groundwater in nature areas. The map is based on measurements of the TrendMeetnet Acidification. At 150 locations in the Netherlands, especially in forest/heide areas on sand, samples were taken from the upper meters of the groundwater. The sites were sampled 6 times in the period 1989-2014.